Poly(vinylidene fluoride) polymers have found broad application as insulation for high-temperature wire, for tank linings, in protective paints and coatings, as encapsulants, in films, and for a variety of other uses. More recently, with the development of polymer electrets, interest in poly(vinylidene fluoride) films has grown even greater, particularly since the films have been show to have piezoelectric properties of a high order, as well as to display pyroelectric characteristics.
Commonly, PVDF films have been prepared by a film casting process using a "coat hanger" die, coupled with a second processing stage involving stretching of the film on a tentering frame. The second stage involves a solid state stretching, either uniaxially or biaxially, carried out at a desired temperature below the film's melting temperature. Films produced by the technique described generally exhibit both superior quality and good uniformity. However, the tentering film process has the disadvantage of requiring more initial capital investment than a "blown film process", described in the following, because of the complexity of the equipment required.
The alternative technique of tubular film extrusion, which allows the film being processed to be simultaneously biaxially stretched, i.e., both longitudinally and transversely, was developed some years ago. The technique involves the extrusion of film from a circular die, and its subsequent orientation in a longitudinal direction through "drawing" of the film by means of a film wind-up apparatus. The drawing results due to the fact that the wind-up occurs at a faster speed than the film extrusion rate, the difference being expressed as the "draw down ratio". During the process of extrusion, air is also introduced into the forming tube, expanding its transverse cross-section, thus providing orientation transverse to the film's longitudinal axis, an expansion known as the "blow-up ratio".
Films produced by the blown film process, however, are generally insufficiently oriented to provide good physical properties. In order to provide adequate orientation during the blown film process, therefore, a second stage solid state stretching is sometimes applied to the initially blown film. This dual stretching operation is termed a "double bubble tubular film process"; and it involves the reheating and reinflation of the tubular film after it has been collapsed following its initial formation.
PVDF can readily be processed by the single bubble blown film process employing a variety of drawn down and blow-up ratios. However, when an attempt is made to additionally stretch the oriented film thus obtained in two directions by means of a second bubble-forming procedure, the film is easily torn and broken during the added stretching. Furthermore, during the second stretching, the resulting film commonly "necks", i.e., its thickness becomes extremely uneven, a phenomenon occurring over the entire surface of the film. In addition, and unlike other crystalline polymers, necking in polyvinylidene fluoride does not show any tendency to disappear, even when the stretching temperature is elevated almost to the melting point of the film.
The described difficulty in further stretching is caused by the high crystallinity of PVDF, its extremely high crystallization rate, and the high intermolecular cohesive force of the molecules.